1. Field of the Invention
The present invention relates to modulation methods and systems for recording digital data on an information medium such as an optical disc, more particularly to modulation methods and systems for minimizing a cumulative digital sum value (DSV) for high-density optical storage systems.
2. Description of the Prior Art
Prior to transmission or recording of digital data, the digital data is usually converted to another data pattern through a certain modulation method. In compact disc (CD) recording systems, the data to be recorded on a compact disc is modulated using EFM (eight-to-fourteen modulation), whereas the data to be recorded on a digital versatile disc (DVD) is modulated using EFM+(eight-to-sixteen modulation). However, during the process of EFM or EFM+modulation, it is important to keep the DSV value as close to zero as possible to allow reliable tracking and reliable detection of high frequency signals. Therefore, there were several methods proposed for keeping the absolute value of the DSV as low as possible to suppress DC (direct current) content during a modulation procedure. In advanced optical discs such as Blu-ray disc (BD), and high density DVD (HD-DVD), to improve the ability of suppressing DC (direct current) content during the modulation procedure, DC (DSV) control bits are included in 17PP modulation for BD and eight-to-twelve modulation for HD-DVD. By controlling the values of DC control bits, the absolute value of the cumulative DSV could be kept as low as possible to suppress DC content during the modulation procedure.
FIG. 1 shows a functional block diagram of an 8-16 (EFM+) modulation system for transforming 8bit data word B(t) into 16-bit code words X(t). During the modulation procedure, each 8-bit data word B(t) associated with a current state S(t) is converted to a main code word Xm(t) having 16 channel bits and a main next state Sm(t+1) through a main conversion table 11. If the data word B(t) is less than eighty-eight, a comparator 13 enables a substitution conversion table 12 to simultaneously output a sub code word Xs(t) and a sub next state Ss(t+1). Meanwhile, a DSV controller 14 is also enabled to calculate a DSV corresponding to each of the main and sub code words, and select one of the code words to be the output code word X(t). The code word X(t) is selected so as to minimize the absolute value of the cumulative DSV. If the main code word Xm(t) is selected as the code word X(t), the main next state Sm(t+1) is designated as the next state S(t+1). The next state S(t+1) is temporarily stored in the state register 15. Similarly, if the sub code word Xs(t) is selected as the code word X(t), the sub next state Ss(t+1) is designated as the next state S(t+1). For DVD modulation, a code word X(t) is obtained through the aforesaid conversion tables 11 and 12 when a data word B(t) and the corresponding current state S(t) are known. The code word X(t) is independent from subsequent data word B(t+1).
During the modulation process for a high density optical storage system such as HD-DVD, Blu-ray, or AOD system, a modulation code word is obtained after determination of the DSV control bit. A DSV control bit may not exist in each data word, and ideally, the determination of the DSV control bits should depend upon all the data words so that the overall cumulative DSV is kept to the minimum. Consequently, a large number of registers is required for storing data words or code words during the modulation process, which also result in a long latency delay.